Sensor system for detecting variables to measured on a rotating object

ABSTRACT

A sensor system for detecting at least one dimensional variable of a rotating object ( 30 ) includes a plurality of sensors ( 33 ) disposed on the rotating object ( 30 ) that are sensitive to the dimensional variable and an antenna array ( 11 ) for supplying the sensors ( 33 ) with high-frequency energy and for receiving a high-frequency signal, modulated by the variable to be detected, from the sensors ( 33 ). The sensors are disposed on the object ( 30 ), distributed in the circumferential direction, and the antenna array ( 11 ) has a directional characteristic ( 34 ) for transmission and/or reception which is stationary with respect to a coordinate system not rotating with the object ( 30 ) and which includes only a subregion ( 32 ) of the object ( 30 ).

[0001] The present invention relates to a sensor system for remotedetection of at least one dimensional variable, which is measured at arotating object.

PRIOR ART

[0002] The capability of remote polling of a sensor is necessary in manykinds of application, especially wherever it is problematic to establisha permanent physical connection between a sensor and an associatedevaluation unit, by way of which connection output signals of the sensorcan be transmitted to the evaluation unit. Such connection problemsarises wherever the sensor is moved relative to the associatedevaluation unit, especially when rotary motions are involved. Examplesof this that can be given are detecting the pressure in a pneumatic tiremounted rotatably on a vehicle, or measuring the torque on a rotatingshaft.

[0003] These applications require the transmission of output signals ofthe sensor electromagnetically, in the broadest sense; that is, thetransmission of radio signals, microwave signals, or light signals. Onepossibility of doing so is to equip the sensor element with its ownpower supply, to furnish the energy needed for the measurement and fortransmitting the output signals. However, this principle quickly reachesits limits because of the attendant costs (battery), the relatively highweight of the sensor unit, and the requisite maintenance, since thebattery has to be replaced after a certain time in operation.

[0004] It is therefore desirable to make the sensor entirely passive, orin other words to embody it without its own power supply, in order tocircumvent the problems associated with the battery and to make thesensor smaller, lighter in weight, and less vulnerable.

[0005] One example of a sensor system with sensors that can be remotelypolled electromagnetically is discussed in German Patent DE 19 702 768C1. The sensor system known from this reference includes the following:

[0006] a sensor, disposed on the rotating object and sensitive to thedimensional variable, and means for forwarding the signals of the sensorto a processing device, which means include an antenna array forsupplying the at least one sensor with high-frequency energy and forreceiving a high-frequency signal, modulated as a function of thevariable to be detected, from the sensor.

[0007] This sensor system is suitable for detecting dimensionalvariables of the rotating object that are essentially constantthroughout the entire object, so that the precise location where ameasurement is made is not critical.

[0008] However, if it is critical to detect dimensional variables whosevalues are not uniform throughout the object, then the known sensorsystem rapidly reaches its limits. Measurements can still be performedin subregions of the rotating object if these subregions rotate jointlywith the object, or in other words if the sensor can be disposed at thesubregion of interest and can rotate jointly with it; but if it iscritical to detect dimensional variables in a subregion of the rotatingobject that is stationary relative to a coordinate system that does notrotate with the object, then the known system is taxed beyond itscapabilities.

ADVANTAGES OF THE INVENTION

[0009] By means of the present invention, a sensor system for detectingat least one dimensional variable of a rotating object is created thatmakes it possible in a simple way to detect a dimensional variable in asubregion of the rotating object that is stationary with respect to acoordinate system that does not rotate with the object.

[0010] These advantages are attained by providing that a plurality ofsensors are disposed on the object, distributed in the circumferentialdirection, and that the antenna array has a directional characteristicfor transmission and/or reception which is stationary with respect to acoordinate system not rotating with the object and which includes onlythe subregion of the object.

[0011] In the course of the rotation of the object, many of the sensorsdisposed on it move successively through the subregion, where they cancome to interact with the antenna array. This means that only when theaffected sensors are located in the subregion are they supplied withhigh-frequency energy that enables them to broadcast an answering radiosignal, and/or that an answering radio signal broadcast by the sensorswill be received by the antenna array only when the affected sensor islocated in the subregion.

[0012] The subregion can advantageously be a contact face of the object,with a substrate. It is then possible for instance to measure contactforces that are operative between the object and the substrate while theobject is rolling over the substrate.

[0013] To keep the system simple and compact, it is preferred that theantenna array includes a common antenna for both sending high-frequencyenergy to the sensors and receiving an answering signal from thesensors.

[0014] Sensors that are used to detect the same physical variable canexpediently have a spacing in the circumferential direction of theobject that is essentially equivalent to the length of the subregion inthe circumferential direction. In this way, it is assured that over thecourse of the rotation of the object, there is one sensor for theapplicable dimensional variable located in the subregion at all times,so that a continuous measurement of the dimensional variable is assured.

[0015] It is especially preferable that the sensors have coding, whichmakes it possible to supply high-frequency energy selectively to atleast one sensor from among a plurality of sensors located in thesubregion, or to receive selectively from at least one sensor located inthe subregion. Such a provision makes it possible to stagger the sensorson the circumference of the rotating body closer together than would beequivalent to the length of the subregion in the circumferentialdirection; since the sensors can be polled selectively, however, thedimensional variable can be determined with a local resolution that isfiner than the length of the subregion.

[0016] Especially easy identification of the sensors is provided for itthe sensors form n groups, which are each distributed cyclically overthe circumference of the object.

[0017] To assure an unambiguous association of the measured values withthe various sensors furnishing them, it is preferred that the subregionis defined such that the sensors of all n groups are never all locatedin it at the same time.

[0018] It is also expedient that each sensor has a first resonator,which can be excited by a modulated measurement frequency of a carrierfrequency of the high-frequency energy, and whose resonant frequency isvariable as a function of the dimensional variable. This resonantfrequency may be modulated to an answering radio signal, which thesensor sends to the antenna array, so that from the modulationfrequency, a processing device connected to the ant can conclude whatthe value of the dimensional variable to be detected is.

[0019] This resonator preferably includes a surface wave resonator or aquartz oscillator as an element capable of oscillation. Also, a discretecomponent that is sensitive to the dimensional variable is preferablyalso incorporated into the first resonator, which makes it possible touse economical standard components as the element capable ofoscillation.

[0020] The use of resonators with a resonant frequency that is variableas a function of the dimensional variable also has the advantage thatthe aforementioned coding can be achieved by assigning each sensor inthe sensor system its own specific resonator tuning range. This makes itpossible, on the basis of the modulation frequency of the answeringradio signal, arriving at the antenna array from a sensor, to concludewhat the identity of the transmitting sensor is.

[0021] If the tuning ranges of the individual first resonators ofvarious sensors partly overlap, then an association of the receivedanswering radio signal with a sensor can be made taking into account thereception field intensity at the antenna array as well. A simplerassociation is obtained, however, if the resonator tuning ranges of theindividual codings are disjoint.

[0022] A preferred application of the sensor system of the invention isthe detection of vectorial variables, in particular forces oraccelerations. If for instance the rotating object is a vehicle tire,then detecting such variables makes it possible to detect dangeroussituations, such as aquaplaning, inadequate adhesion of individualwheels of the vehicle in cornering, and so forth, and to generate awarning to the vehicle driver accordingly.

[0023] In such a case, it is often not necessary that all threecomponents of the vectorial variable be detected; in the instances givenabove, it is sufficient if the sensors are each designed for detectingtwo components of the dimensional variable that are perpendicular to oneanother and tangential to the surface of the object. A conclusion as tothe value of a component of the vectorial variable that is orientedradially to the tire can be drawn by measuring the tire pressure, forinstance.

[0024] It is also advantageous that each sensor has a second resonatorthat can be excited by a carrier frequency of the high-frequency energy.This second resonator makes it possible to store the high-frequencyenergy for a limited time, so that it is available for generating theanswering radio signal. This has the advantage, first, that the sensorfor generating the answering radio signal need not rely onsimultaneously transmitting the high-frequency energy through theantenna array, because during a pause in the supplying of high-frequencyenergy, the second resonator is capable of furnishing the energyrequired for sending the answering radio signal. Since there can bepauses in the supplying of the high-frequency energy, it is convenientlypossible to use the same antenna, at staggered times, both to supply thehigh-frequency energy to the sensors and to receive their answeringradio signal. Thus the first resonator makes it possible to constructthe sensor as a passive element, without its own power supply.

[0025] A further advantage of using the second resonator is that itenables selective excitation of individual sensors by means of a pollingradio signal, with a carrier frequency tuned to the second resonator,or, in an environment in which at lead one sensor each is assigned to aplurality of polling units, it enables each polling unit and itsassigned sensors to be allocated a specific carrier frequency thatenables the polling units to answer and poll its assigned sensorsselectively.

[0026] As the second resonator, surface wave resonators are especiallypreferred.

[0027] Surface wave resonators of the kind that are capable ofgenerating a delayed output oscillation pulse in reaction to an inducedoscillation pulse are especially advantageous. During a first timeinterval that is meant to be shorter than the delay in the secondresonator, such resonators can be excited to an oscillation; the energystored in this oscillation, however, is not available to the sensor asdriving energy until there is a pause in the high-frequency energysupply through the antenna array. As long as the delay persists, theenergy is stored in the second resonator with only slight losses,dictated by the oscillation damping of the resonator substrate.

[0028] Such a delay can be achieved easily with the aid of a propagationdistance for the surface wave that a surface wave induced in the secondresonator must traverse before it is picked up.

[0029] Such resonators can be embodied for instance as surface wavefilters, with a first pair of electrodes for inducing the surface waveand with a second pair of electrodes, three-dimensionally spaced apartfrom the first, for picking up the surface wave; the two pairs ofelectrodes can be separated from one another by the propagationdistance.

[0030] Alternatively, they can be embodied as resonators with a singlepair of electrodes, which then serves both to induce and to pick up thesurface wave; reflector electrodes are each spaced apart from the firstpair of electrodes, for reflecting the surface wave, being propagated inthe resonator substrate, with a time lag relative to the first pair ofelectrodes.

[0031] Further characteristics and advantages of the invention willbecome apparent from the ensuing description of exemplary embodiments inconjunction with the appended drawings.

DRAWINGS

[0032] Shown are

[0033]FIG. 1, a schematic illustration of a vehicle wheel that isequipped with a sensor system of the invention, in a first exemplaryembodiment;

[0034]FIG. 2, a schematic illustration of a vehicle wheel that isequipped with a sensor system of the invention, in a second exemplaryembodiment;

[0035]FIG. 3, a block diagram of a sensor of the vehicle wheel of FIG.1;

[0036]FIG. 4, a block diagram of a polling unit for the sensor of FIG.2;

[0037]FIG. 5, the course over time of the intensities of the radiosignals at the antenna array of the polling unit of FIG. 3;

[0038]FIG. 6, a first example for the makeup of a surface wave resonatorthat is suitable as a second resonator for a sensor such as the sensorof FIG. 2;

[0039]FIG. 7, a second example for the makeup of a surface waveresonator; and

[0040]FIG. 8, the course over time of the intensities of the radiosignals at the antenna array when a second resonator of the type shownin FIG. 5 or FIG. 6 is used.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0041]FIG. 1 shows a first example of a vehicle wheel, with a pneumatictire 30 that is equipped with a sensor system of the invention. Manysensors 33 are disposed in the running surface of the pneumatic tire;they can for instance be embedded in tread elements of the pneumatictire, or in the region of the (steel) jacket.

[0042] The sensors 33 may be capacitive or inductive sensors, whosemakeup and mode of operation will be addressed in further detailhereinafter in conjunction with FIG. 3.

[0043] The sensors 33 are intended for measuring the deformation of theprofile of the pneumatic tire 30 in a subregion of the pneumatic tire30, namely in its flattened contact face 32 with the road.

[0044] An antenna 11 is disposed in the vicinity of the axle of thewheel and has a directional characteristic oriented toward the flattenedregion 32; this directional characteristic is represented here by thelobe 34.

[0045] The antenna 11 is part of a polling unit, which is shown in FIG.4 in the form of a block diagram. An oscillator 13 is located in thepolling unit and generates a signal, here called the polling carriersignal, with a carrier frequency f_(T) in the range of 2.54 GHz. Thecarrier frequency is preferably purposefully variable by several MHz. Asecond oscillator 14 generates a polling measurement signal in the formof an oscillation with a frequency f_(M) in the range from 0 to 80 MHz.When the polling unit is used to poll a plurality of sensors, themeasurement frequency f_(M) is expediently also variable purposefully,specifically in increments that correspond to the size of the resonantrange of a first resonator of the sensors, which will be addressed infurther detail hereinafter.

[0046] A modulator 15 is connected to the two oscillators 13, 14 andmodulates the polling measurement signal to the polling carrier signaland thus generates a polling radio signal that is output to a switch 12.The switch 12 is under the control of a timer 15, which alternatelyconnects a sending and receiving antenna 11 with the output of themodulator 14 and the input of a demodulation and measuring circuit 17,which acts as a processing device for extracting the values of thedimensional variables to be detected from the answering radio signalsreceived. The modulation performed by the modulator 15 can in particularbe amplitude modulation or quadrature modulation; the demodulationperformed in the demodulation and measuring circuit 17 is complementaryto it.

[0047] The makeup of the sensors 33 is shown in FIG. 3 in the form of ablock diagram. The polling radio signal broadcast by the antenna 11 isreceived by an antenna 1 of the sensor shown in FIG. 3. A demodulationdiode 2, such as a Schottky or detector diode, is connected to theantenna. Such a diode is distinguished by an essentially paraboliccharacteristic curve even in the vicinity of the origin in thecoordinate system and thus by a highly nonlinear behavior, which leadsto mixing of the spectral components contained in the polling radiosignal, and thus to the generation of a spectral component with thefrequency f_(M) of the measurement signal at the output of thedemodulation diode 2. The spectral component at the carrier frequencyf_(T) that also appears at the output of the demodulation diode 2 servesto excite a resonator 3, in this case called the second resonator.

[0048] A low-pass filter 4, and downstream of the low-pass filter 4, aso-called first resonator 5 are connected to the output of thedemodulation diode 2; together with an element 6 sensitive to thedimensional variable, this first resonator forms an oscillating circuit.The first resonator 5, just like the second resonator 3, is acommercially available component, such as a quartz oscillator or asurface wave resonator. By means of the interconnection with thesensitive element 6, the resonant frequency of the first resonator 5 isvariable as a function of the dimensional variable.

[0049] The purpose of the low-pass filter 4 is essentially to keepspectral components in the range of the carrier frequency f_(T) awayfrom the first resonator 5 and thus to prevent their dissipation in thefirst resonator 5. In this way, the low-pass filter 4 on the one handaccomplishes more-effective excitation of the second resonator 3, aslong as the polling radio signal is being received by the antenna 1;when there is a pause in the polling radio signal, the low-pass filter 4also limits the attenuation of the second resonator 3.

[0050] The sensitive element 6 is an inductive or capacitive element,such as a micromechanical pressure sensor element with two capacitorplates movable relative to one another as a function of an exerted forceor acceleration. Such an element 5 essentially affects only the resonantfrequency but not the attenuation of the first resonator 5.

[0051] Since such a sensor is sensitive only to a force or accelerationcomponent in one direction in space, three sensors 33 are provided atevery circumferential position 31 on the pneumatic tire 30 of FIG. 1:two sensors for the directions tangential to the surface of thepneumatic tire, one being in the direction of vehicle travel and theother being transverse to it, and a third sensor for the radialdirection.

[0052]FIG. 5 schematically illustrates the course of the reception fieldintensity P at the antenna 11 of the polling unit as a function of thetime t in the course of one polling cycle. The reception field intensityP is plotted on a logarithmic scale. During a period of time from t=0 tot=t₁, the polling radio signal is broadcast and is thus necessarily someorders of magnitude stronger than the echo signals thrown back from theenvironment of the polling unit or than an answering signal possiblyfurnished by a sensor.

[0053] At time t₁, the switch 12 connects the antenna 11 with thedemodulation and measuring circuit 17 and the broadcasting of thepolling radio signal is interrupted. During a brief period of time [t₁,t₂], echos of the polling radio signal arrive at the antenna 11, havingbeen thrown back by obstacles various distances away in the environmentof the antenna 11.

[0054] Once these echo signals have faded, the only signal that thenarrives at the antenna 11 is an answering radio signal, which has beengenerated in the sensor 33 by mixing of the oscillations of the tworesonators 4, 5 by the diode 2 functioning as a modulator and has beenbroadcast via the antenna 1. The demodulation and measuring circuit 17therefore waits out a predetermined length of time Deltat after theswitchover of the switch 12 before beginning to examine the answeringsignal received at the antenna 11 as to its frequency and/or attenuationand thus to extract the information it contains about the dimensionalvariable.

[0055] The delay Deltat can be fixedly specified as a function of thetransmission and reception power of the polling unit, for example insuch a way that for a given model of polling unit, a maximum range atwhich echo signals are still detectable by the polling unit isdetermined, and the delay Deltat is selected to be at least twice thetransit time that is equivalent to this range.

[0056] Since during the delay period Deltat the oscillations of theresonators 3 and 5 also face, however, it is more advantageous to selectas short as possible a delay period Deltat as a function of theparticular environment in which the polling unit is used; for examplefor a specific usage environment, the maximum distance of a potentialecho source from the polling unit is determined, and the delay isselected to be at least equal to twice the signal transit time from thesensor element to the polling unit, and thus precisely long enough thatan echo from that source will not be evaluated. In the sensor systemshown in FIG. 1, the time equivalent to the transit time of a radiosignal from the antenna 11 to the roadway in the vicinity of theflattened region 32 and back to the antenna 11 again can be selected asthe delay period Deltat.

[0057]FIG. 8 schematically illustrates the course of the reception fieldintensity P at the antenna 11 of the polling unit as a function of thetime t in the course of one polling cycle that results when a surfacewave resonator of the design shown in FIG. 6 or FIG. 7 is used as thesecond resonator of the sensor.

[0058] Just as in the case of FIG. 5, the polling radio signal isbroadcast during a time period from t=t₀ to t=t₁. At time t₁, thebroadcasting of the polling radio signal is interrupted; the receptionfield intensity P at the antenna 11 decreases to the extent that echosof the polling radio signal that are thrown back from the environment ofthe antenna 11 fade.

[0059] At time t₃=t₁+T (ignoring signal transit times between thepolling unit and the sensor), the surface wave, which has been inducedin the second resonator 3 by the sensor during the reception of thepolling radio signal, begins to reach the pair of electrodes at which itis picked up, so that from time t₃ on, a modulated answering radiosignal is generated at the sensor. Because the length of the secondresonator 3 or the delay τ within this resonator 3 is selected to begreat enough, it is possible to achieve a pause in reception, betweenthe fading of the echos at time t₂ and the arrival of the answeringradio signal at time t₃, of negligible reception field intensity, whichis detectable by the demodulation and measuring circuit 17 of thepolling unit and allows the polling unit to distinguish unambiguouslybetween an echo and an answering radio signal. At time t₄=t₁+τ, thesurface wave oscillation has completely traversed the electrode pairthat is picking up the signal, and the generation of the answering radiosignal ceases.

[0060] After a brief further delay, upon renewed broadcasting of thepolling radio signal at time t₅, a new work cycle of the polling unit ofthe sensor begins.

[0061]FIG. 2 shows a more sophisticated embodiment of the sensor systemof FIG. 1. Here only two sensors 33 are disposed at each position 31 onthe circumferential surface of the tire, and each sensor is sensitive toeither force or acceleration in the directions tangential to the surfaceof the pneumatic tire. Their makeup is the same as described above inconjunction with FIG. 1, 3, 5 or 6.

[0062] The sensor, sensitive to a force or acceleration in the radialdirection, located at each position 31 in FIG. 1 is replaced here by asingle sensor 36, which measures the dynamic internal pressure of thepneumatic tire. From this internal pressure, or its changes, aconclusion can be drawn as to the force acting on the pneumatic tire 30in the radial direction. This sensor 36 has an antenna 37 or antennaarray, which extends to some length along the circumferential directionof the pneumatic tire and one part of which, in every rotationalposition of the pneumatic tire, is located inside the lobe 34 of theantenna 11, so that the pressure sensor 36 can be polled at anyarbitrary instant.

[0063] The single pressure sensor 36 thus replaces all the sensors forthe force or acceleration in the radial direction of the exemplaryembodiment of FIG. 1. This makes a considerable reduction in the numberof sensors possible, compared to the exemplary embodiment of FIG. 1. Forinstance, assuming a circumference of the pneumatic tire 30 of about 2meters and a spacing between positions 31 of the individual sensors ofabout 10 cm, the number of sensors required is reduced from 3×20=60 inthe case of the exemplary embodiment of FIG. 1 to 2×20+1=41 in the caseof FIG. 2.

[0064] In the exemplary embodiments shown in FIGS. 1 and 2, the lengthof the lobe 34 in the circumferential direction and the spacing of thesensor positions is selected such that at every instant, three positions31 are located inside the lobe 34. This means that at every instant,either nine or seven sensors (six sensors 33 for the tangentialdirections, and the pressure sensor 36) can be addressed by the pollingradio signal of the antenna 11. For usable remote polling, it isnecessary to be able to distinguish between the answering radio signalsthat originate at a plurality of sensors disposed at the same positionand the answering radio signals furnished by sensors at differentpositions 31. To that end, coding of the radio signals is necessary.Software coding is not appropriate here, first because of the processingtimes associated with executing a program and second because the sensorscan obtain the energy required for such coding only from the pollingradio signal, and such energy is scarce.

[0065] Coding with the aid of the carrier and measurement frequencies ofthe radio signals exchanged between the antenna 11 and the sensors istherefore employed. The sensors 33 distributed over the circumference ofthe pneumatic tire 30 are each divided up into a plurality of groups; inthe examples of FIGS. 1 and 2 this number of groups has been arbitrarilydefined as four, and depending on which of the four groups their sensors33 belong to, the positions 31 are each marked a, b, c, or d in FIGS. 1and 2.

[0066] In a first variant, the carrier frequency f_(T) of the pollingradio signal is the same for all of the sensors 33, and the secondresonators 3 of all the sensors 33 are tuned to this carrier frequencyf_(T). The first resonators have tuning ranges that differ within agroup depending on the dimensional variable to be detected by the sensor33 and that moreover differ from one group to another. In the case ofFIG. 2, for example, if it is assumed that the tuning ranges of thefirst resonators 5 each have a width of 1 MHz, then the followingassociation of tuning ranges with groups and dimensional variables canbe made:

[0067] Group Force in travel direction Force in transverse direction a 0-10 MHZ 40-50 MHZ b 10-20 MHZ 50-60 MHZ c 20-30 MHZ 60-70 MHZ d 30-40MHZ 70-80 MHZ

[0068] The polling unit is thus capable, by selecting the measurementfrequency, of selectively exciting only the first resonators of ongroup, and within this group only the first resonators of the sensor 33that is associated with a certain dimensional variable, so that theanswering radio signal received following this excitation can only comefrom the sensor 33 thus addressed.

[0069] It is understood that it is also possible to modulate a pluralityof measurement frequencies to the answering signal, so that answeringradio signals are received simultaneously from a plurality of sensors33, and the measurement frequencies of the answering radio signals thatoverlap chronologically can be broken down spectrally in the pollingunit, so as to associate them with the individual sensors 33, or thedimensional variables monitored by them.

[0070] Another possibility is to assign different carrier frequencies,in the same tuning ranges of the first resonators 5, to various sensors33 disposed at the same position 31 and belonging to the same group. Inthis way, from each of these sensors answering radio signals can bereceived which while they have the same measurement frequencies, or moreprecisely measurement frequencies within the same tuning range, arenevertheless distinguishable from one another in the polling unit on thebasis of their different carrier frequencies and can thus be associatedcorrectly with the dimensional variables to be detected.

[0071] If two dimensional variables are to be detected at one position31, it may also be expedient to construct the antennas 1 of the sensors33 with polarization sensitivity. For instance, the antenna 1 of asensor 33 that detects a force in the travel direction can be sensitiveonly to a polling radio signal polarized parallel to the traveldirection, and a sensor 33 disposed at the same position 31 fordetecting the force transversely to the travel direction is sensitive toa polling radio signal polarized transversely to the travel direction.The polarizations of the answering radio signals broadcast by the twosensors 33 differ correspondingly, so that the polling unit candistinguish the answering radio signals from their polarization.

[0072] While the vehicle is in motion, all the sensors of the wheelshould be polled continuously. To that end, in a simple embodiment, thelobe 34 of the antenna 11 can be dimensioned such that essentially onlyone position 31 at a time is ever located inside the lobe 34. To avoidinterference with the dimensional variable from sensors located at theperiphery of the lobe 34, a very sharply defined spatial boundary of thelobe 34 is necessary in that case.

[0073] In an advantageous alternative in this respect, the size of thelobe 34 of the antenna 11 in the circumferential direction of thepneumatic tire 30 is on the one hand large enough that a plurality ofpositions 31 are always located inside this lobe 34, yet on the otherhand not so large enough for there to be room in it for the sensors ofall the groups. In the position of the wheel shown in FIG. 1 and FIG. 2,the polling unit can excite sensors in each of the groups c, d and a andreceive answering radio signals from them, but sensors of group b arenot located in the lobe 34. Since the groups a, b, c, d follow oneanother cyclically, the polling unit can conclude from the absence of ananswering radio signal from group b that the sensors of groups a and cmust be located in the vicinity of the edge of the flattened region 32,and that the sensor of group d must be located in the middle of theflattened region 32. At the edge of the region 32, there is a strongflexing motion on the part of the material comprising the pneumatic tire30, and as a result the sensors of groups a and c can be subjected topowerful forces. The sensor of group d, conversely, must be located inthe middle of the flattened region 32, i.e. at the place where theflexing motion is only slight, yet the transmission of force between thepneumatic tire 30 and the roadway is most effective. The answering radiosignal furnished by this sensor thus makes it possible to draw theprecisest possible conclusion about the quality of road adhesion of thepneumatic tire. The polling unit therefore identifies the answeringradio signal of the sensor of group d from its characteristicmeasurement frequency and for instance causes a warning signal to beoutput to the vehicle driver if the instantaneous value of thismeasurement frequency, which represents the force detected by the sensorin group d, departs from a desired range. In this way the driver can bewarned even before the road adhesion of the vehicle is lost, forinstance from aquaplaning or traveling on an icy surface, and the riskof accidents can be reduced.

1. A sensor system for detecting at least one dimensional variable of arotating object (30), having at least one sensor, disposed on therotating object (30) and sensitive to the dimensional variable, andhaving means for picking up measurement signals from the at least onesensor and forwarding the signals to a processing device, which includean antenna array (11) for supplying the at least one sensor withhigh-frequency energy and for receiving a high-frequency signal,modulated as a function of the variable to be detected, from the sensor,characterized in that a plurality of such sensors are disposed on theobject (30), distributed in the circumferential direction; and that theantenna array (11) has a directional characteristic (34) fortransmission and/or reception which is stationary with respect to acoordinate system not rotating with the object (30) and which includesonly a subregion (32) of the object (30).
 2. The sensor system of claim1, characterized in that the subregion is the contact face of the object(30), with a substrate.
 3. The sensor system of claim 1 or 2,characterized in that the antenna array (11) includes a common antennafor transmission and reception.
 4. The sensor system of one of theforegoing claims, characterized in that sensors used to detect the samephysical variable have a spacing in the circumferential direction of theobject (30) that is essentially equivalent to the length of thesubregion (32) in the circumferential direction.
 5. The sensor system ofone of the foregoing claims, characterized in that the sensors havecoding, which makes it possible to supply high-frequency energyselectively to at least one sensor from among a plurality of sensorslocated in the subregion (32), or to receive selectively from at leastone sensor located in the subregion.
 6. The sensor system of claim 5,characterized in that the sensors form n groups, which are eachdistributed cyclically over the circumference of the object (30).
 7. Thesensor system of claim 6, characterized in that the subregion is definedsuch that the sensors of all n groups are never all located in it at thesame time.
 8. The sensor system of one of the foregoing claims,characterized in that each sensor has a first resonator (5), which canbe excited by a modulated measurement frequency of a carrier frequencyof the high-frequency energy, and whose resonant frequency is variableas a function of the dimensional variable.
 9. The sensor system of claim8, characterized in that the first resonator (5) includes a surface waveresonator or a quartz oscillator.
 10. The sensor system of claim 9,characterized in that the first resonator (5) further includes adiscrete component (6) that is sensitive to the dimensional variable.11. The sensor system of one of claims 5-7 and one of claims 8-10,characterized in that one specific resonator tuning range corresponds toeach coding.
 12. The sensor system of claim 11, characterized in thatthe resonator tuning ranges of the individual codings are disjoint. 13.The sensor system of one of the foregoing claims, characterized in thatthe dimensional variable is a vectorial variable, and in particular aforce or acceleration.
 14. The sensor system of claim 13, characterizedin that the sensors are each designed for detecting two components ofthe dimensional variable that are perpendicular to one another andtangential to the surface of the object.
 15. The sensor system of one ofthe foregoing claims, characterized in that the object (30) is apneumatic tire.
 16. The sensor system of claim 15, characterized in thatit also has an individual sensor (36) for the tire pressure.
 17. Thesensor system of one of the foregoing claims, characterized in that eachsensor has a second resonator (3) that can be excited by a carrierfrequency of the high-frequency energy.
 18. The sensor system of claim17, characterized in that the second resonator (3) is a surface waveresonator.
 19. The sensor system of claim 18, characterized in that thesecond resonator (3) is capable of generating a delayed outputoscillation pulse in reaction to an induced oscillation pulse.
 20. Thesensor system of claim 19, characterized in that the second resonator(3) has a propagation distance (L) for the surface wave that a surfacewave induced in the second resonator (3) must traverse before it ispicked up.
 21. The sensor system of claim 19 or 20, characterized inthat the second resonator has two three-dimensionally spaced-apart pairs(25, 236) of electrodes (21, 22).
 22. The sensor system of claim 19 or20, characterized in that the second resonator has one pair (27) ofelectrodes (21, 22), for inducing and picking up a surface wave, andreflector electrodes (23), spaced apart from the pair (27) ofelectrodes.